The present invention relates to an optical disc apparatus for recording and reading information to and from an optical disc.
Periodic external disturbance caused by rotation of an optical disc is one factor causing a deviation in servo tracking performed in an optical disc apparatus. A known method of improving the capability to follow such periodic disturbance is iterative learning control (see, for example, JP-A-9-50303).
However, according to JP-A-9-50303, in the iterative learning control system, a control output is created for compressing rotation-synchronized vibrations using a position error signal produced one rotation earlier. It is known that during one rotation while the iterative learning control is in operation, the iterative learning control produces no effect.
Therefore, there is the problem that the repetitive compensator produces no action until a control error signal corresponding to one rotation of disc is accumulated in a memory inside the repetitive compensator whenever the mode of operation is switched to tracking control mode immediately after a seek operation. In an attempt to solve this problem, JP-A-2003-249045 discloses a method in which a repetitive compensator continues learning using values learned during tracking control prior to start of a seek operation as initial values immediately after a tracking operation is resumed subsequent to completion of the seek operation.
It is known that periodic external disturbances produced when an optical disc rotates include a component known as a deviation, in addition to eccentric and surface wobble components. JP-A-2007-207390 sets forth, “An optical disc having guide grooves may sometimes have ill-shaped groove portions (deviations) due to deterioration of the disc stamper or inferior formation of the disc. Where such a disc is rotated at a high multiple speed, wide-band peculiar noise components are introduced into the groove-reflected signal, especially near the outer periphery.”